1. Field of the Invention
This invention relates to a method and system for digital transmission of pulses of predetermined rate or pulse width. In particular, this invention relates to a method of transmitting a data stream from a transmitter wherein said data stream has a narrow band signal spectrum. More in particular, this invention relates to a system where digital data to be transmitted is modulated in the frequency domain resulting in a substantially rectangular signal spectrum which is reconverted in a receiving unit into a data stream. Still further, this invention relates to recovery of a data stream being transmitted through demodulation of a narrow band signal spectrum output from a transmitter and using periodical zeros of an auto-correlation function of the narrow band signal spectrum while limiting the effective duration of the auto-correlation function.
2. Prior Art
Methods and systems for digital data transmission are known in the prior art. In some prior art systems, in order to transmit digital data to radio transmitters or cable, modulation techniques are used to place the transmission into specific frequency ranges. Due to the fact that the data is generally discontinuous, such results in signal spectra which have a slow decay with increasing distance from a center frequency.
In prior art systems and methods for digital data transmission, conventional modulation techniques with regard to bandwidth or power requirements fall far short of the theoretical limits attainable. In some prior art systems where bandwidth considerations are optimized, the ratio of energy/bit (Eb) to spectral noise power (No), which is the signal to noise ratio is extremely inefficient. Opposingly, where Eb/No considerations are optimized, such has been accomplished at the expense of a greater bandwidth.
In some prior art system and method techniques, an attempt was made to reduce the required bandwidth using the concept of optimizing the shapes of the data pulses within the bit duration. This has led to an improved spectral decay at frequencies displaced from the center frequency, however, such improved spectral decay was obtained at the expense of an increased bandwidth close to the center frequency. Such prior art methods and systems are known as minimum shift keying (MSK) as well as various pulse formed versions of quadrature phase shift keying (QPSK). In such prior art systems, symbol duration was considered to be by necessity shorter or equal to the bit duration, and thus, an extended spectra for MSK and variants of phase shift keying was previously in such prior art systems and methods deemed acceptable.
In other prior art systems, such as those known as correlative coding, partial response signalling, and some form of Nyquist filtering, there have been made attempts to reduce the bandwidths by symbol expansion beyond a singular bit length. However, this has necessitated a symbol separation by using a suitable linear combination of adjacent symbols. Such prior art systems provided for the disadvantage of a deterioration of the Eb/No. Thus, due to the low ratio of complexity vs. performance, such prior art methods and systems have not been used extensively.